1. Field of the Invention
This invention resides in the field of surface topographical characterization and more particularly relates to a mechanico-electro-optical apparatus and methods of use thereof.
2. Description of the Prior Art
The topography of a material can often affect its usefulness. Surface reactions and interactions of a variety of materials have been shown to vary with roughness. Many examples are cited in the literatures of different fields, relating a variety of phenomena and end use properties to topographic characteristics. These include the literatures of mechanics, fracture mechanics, fluid mechanics, optics, coating, adhesion, machine processing and the biomedical sciences. In addition, the topography of materials has been shown to affect measurements of other surface properties, e. g., contact angle goniometry and reflective spectroscopy.
The importance of topography has therefore resulted in the development of a variety of instruments and procedures to characterize and/or visualize material surfaces. These include instruments for profilometric measurements by stylus and optical means, sequential profilometry by stylus and optical procedures, light and scanning electron microscopy, surface light scattering, thermography and analyses of surface images obtained under angular illumination. A general review of techniques used for quantitative topographic characterization is found in Rough Surfaces, ed.
by T. R. Thomas, Longman, N.Y., 1982. Instruments devised for these purposes are described in U.S. Pat. Nos. 3,319,463; 3,322,978; 3,336,833; 3,379,059; 3,543,571; 3,744,304; 3,747,395; 3,908,079; 4,005,932; 4,050,294; 4,145,140; 4,334,282; 4,441,812; 4,498,043; 4,714,348; and 4,732,403.
There are many purposes, however, for which instrumentation, presently known in the art, does not provide sufficient quantitative information or cannot be used on certain materials. For example, while light and scanning electron microscopy are invaluable for surface visualization, their use for dimensional characterization requires specialized time consuming procedures that make quantitative dimensional analyses by these instruments very difficult, if not impossible, to do.
Along the same vein, measurements of the spatial distributions of thermographic data can and are used for topographic characterization. However, this methodology, which assumes a constant proportionality between localized heights above a plane and localized temperature differences between points on the surface and a reference temperature, is not valid when heat conductivitiy is a spatial variable or when the material undergoes appreciable deformation at elevated temperatures. The methodology is thus limited and not general.
The most commonly used methodology for quantitative topographic characterization has historically been stylus profilometry. In this instrumentation, a stylus traverses a surface while its up-down movement, assumed to follow the surface's profile, is recorded by a displacement transducer and recorder as a function of lateral movement. The methodology requires that the stylus be loaded against the test surface to assure contact.
While stylus profilometry provides quantitative measurements of roughness, its deficiencies are well recognized in the literature. These include the following: (1) test surfaces are often deformed both compressively and laterally as the stylus traverses the surface, making it difficult to uncouple deformational effects from topographic characteristics; (2) Resolution must necessarily be compromised against the average pressure exerted by the stylus; smaller tip radii, permitting greater resolution, results in relatively high pressures on the surface, with consequent increased deformation; (3) Use with high fidelity replicating materials is often unreliable because of the compliant nature of these materials--a serious limitation since replicating materials are often necessary to assess the topographic characteristics of inacessible surfaces. In addition data obtained from single profiles, i. e., heights, shapes, peaks, and valleys, frequently do not correspond to the actual peaks, etc. of the surface. This deficiency can be circumvented, in part, by sequential traces, digitization, and subsequent statistical treatment, but the limited lateral resolution of stylus profilometry and its primary deficiencies with regard to data acquisition remain.
The aforementioned problems with contact measurements have resulted in the development of optical, non-contact instrumentation. These methodologies include among others: (1) speckle pattern analyses; (2) light section microscopy-permitting visualization of and measurements from reflected profile images; and (3) interferometric optical procedures that enable measurements of surface features to be made. These methodologies are well known to the art and commercial instrumentation using these methodologies are available. However, each of these procedures have some decided limitations.
Speckle pattern analyses, i. e. analyses of measurements of the spatial distribution of scattered light intensities, yields information about a surface roughness through mathematical models of the experiment. Instrumentation using this methodology is described in U.S. Pat. No. 4,145,140. However, the methodology provides neither direct visualization nor measurements of specific surface features. In addition, the methodology is limited to the characterization of relatively smooth surfaces and is unsuited for characterizing low frequency oscillations.
Light section microscopy, as currently available, has the following limitations. The widths of the observed profiles are small, (i. e. equal to or less than 1000 microns) precluding characterization of oscillations greater than that. The reflected image is not sharp even when focused, a consequence of the widths of the illuminating beam and the effects of diffuse and specular scattering. Visual measurements of peak to valley heights from profile images require subjective estimates with uncertainties ranging from 5 to 20 microns and more. The magnitudes of relief profiles that can be measured are limited by the optics of the system, (in practice, about .+-.100 microns). In addition, single profile lines, as with stylus profilometry, do not provide sufficient quantitative information. In summary, the visualized profiles allow, for the most part, only imprecise determinations of differences in height and distances between oscillations within the profile.
A variation of this methodology is described in U.S. Pat. No. 3,908,079. Here a video camera and analog device is used in combination with a light section microscope to obtain an averaged value of surface oscillations within the microscopic field. This serves as a measure of roughness. While undoubtedly useful, the more descriptive statistical area parameters, requiring discrete field data, cannot be obtained. In addition, surface topographical features cannot be visualized, the value obtained is limited to the microscopic field, and large relief changes cannot be characterized.
Inteference optical systems coupled to a mechanical stage, controlled by computer have recently been used to quantitatively characterize the topography of surfaces. The optical aspects of such a system are described in U.S. Pat. No. 4,732,403. This methodology permits area characterization, has high sensitivity, is non-contact, and allows both visualization and statistical characterization to be made. However, the range of height changes that can be measured is small. The methodology is limited to the characterization of relatively smooth surfaces. The instrumentation cannot detect height changes greater than 15 microns on the outer limit. The methodology is thus not suitable for characterizing many surfaces of interest, e. g. fracture, paper, and biological surfaces that generally have considerably larger height changes.
In contrast to the prior art described above, the present invention addresses itself to the problem of obtaining real, detailed, quantitative dimensional data of surface features by non-contact means, over relatively wide areas, while permitting measurements to be made of both small and large relief changes from 0.25 microns to centimeters. The apparatus and procedures constituting the invention enable measurements to be made automatically of the distances, z, from a reference plane of closely spaced points on a surface as a function of their x,y coordinates in the reference plane. Mappings of surface coordinates z(x,y) are thereby obtained. These, when plotted, yield three dimensional images of surface features that agree very well with scanning electron and light micrographs of the same regions. Mathematical operations on z(x,y) yield any number of descriptive and statistical parameters that cannot readily be obtained by other means for such a wide range of relief characteristics. Finally, the derived statistical parameters and the images seen visually are from the same data set. The calculated parameters depend only on the coordinate measurements and not on theory.
The invention additionally differs from the above described devices in one or more of the following ways. Relatively large areas of varying relief characteristics can be characterized as opposed to single line profiles, by non-contact means. The size of the characterized area is not restricted to the optical field. The data or coordinates obtained permit both the visualization of surface features and dimensional characterization and the obtaining of descriptive statistical parameters from the same region. Both low and high frequency oscillations can be characterized. Any type of material can be analyzed regardless of compliance, and data acquisition can be totally automated.
In addition to topographic characterization, the system can also be used to measure the spatial distributions of the reflectivities of a material, the overall dimensional characteristics of microscopic objects, and the rates of dimensional change of materials, over wide ranges, in real time.